Al-doped SnO2 thin films: impacts of high temperature annealing on the structural, optical and electrical properties

Undoped Tin Oxide (SnO2) and 4, 8, 14 at.% Aluminum (Al) doped SnO2 thin films (hereafter Sn1-xO2:Alx) were fabricated by thermal vacuum evaporation and stacked layer method, respectively. Following this, a detailed investigation of the impacts of Al doping and annealing temperature on the structural, optical and electrical properties of the synthesized films were carried out. The XRD results show that the films possess tetragonal lattice structures and for both doped and undoped SnO2, the relative intensities of the major diffraction peaks increase with annealing temperature. The high temperature annealing improves the crystallinity and reduces the stacking faults of Sn1-xO2:Alx films. UV–Vis–NIR spectrophotometric studies revealed that Sn1-xO2:Alx films annealed at high temperature have good transmittance which can reach up to 87.84% in the visible region. The optical bandgap gradually decreases with higher annealing conditions and lies within the range of 3.36–3.86 eV. Other properties like penetration depth, Urbach energy, strength of electron–phonon interaction and photoluminescence spectra were also studied. Electrical characterization reveals that Al doping content reduces the carrier mobility and carrier concentration while annealing temperature enhances both of these. This can be attributed to the reduced grain boundary scattering and interatomic bonding at higher annealing temperatures and ionized impurities at higher Al concentration. High annealing suppresses the sheet resistance of all the Sn1-xO2:Alx films and the lowest sheet resistance (268.50 Ω/cm2) is observed at 550 °C annealing for the undoped SnO2 films.